When manufacturing 1) an electronic material that is formed in a structure of a single-layer thin film and a multi-layer thin-film and 2) an electronic device that is application of the electronic material, a sputtering device for forming a thin film under vacuum conditions is significant. Methods of forming a thin film generally include vapor deposition, sputtering and CVD (Chemical Vapor Deposition). Above all, sputtering is widely used in various fields, because any material regardless of substrate material can be safely deposited on the substrate with a comparatively simple device without using toxic gases.
A principle of sputtering is generally described hereinafter. Ions in plasma, generated in a vacuum chamber, impinge on a target so as to cause ejection of constituent atoms and molecules from the target surface, thereby forming a thin film with these constituent atoms and molecules deposited on the substrate. The sputtering device is provided with various types of configurations depending on an ionized gas as an impact ion source, generation methods of discharge plasma, types of applied power supply or electrode structures as shown in FIGS. 18 to 24.
Ion-beam sputtering shown in FIG. 18 introduces irradiation ions generated in an ion chamber into a sputtering chamber where sputtering of a target is performed to deposit a thin film. Kaufman ion source with a thermal cathode type or an ECR (Electron Cyclotron Resonance) ion source is employed depending on the methods of generating ions. In either case, sputtering is performed with an Ar ion beam, introduced and irradiated on a target. Even when pressure of discharge is as low as 10−4 Torr or less, sputtering can be performed, and with small discharged gas incorporation into a thin film and a large kinetic energy of sputter particles, precise thin-film formation with superior surface smoothness can be achieved. However, the deposition rate of the thin film is disadvantageously low.
In sputtering shown in FIG. 19 and FIG. 20, plasma ions, accelerated in a cathode fall region, impact a target to cause sputtering, and thus sputtered particles are deposited on the facing substrate, forming a thin film. In FIG. 20, a target unit 3, including a target 1 and a target plate (target holder) 2, is shown. Direct current (DC) sputtering or alternate current (RF) sputtering is employed depending on the applied power supply. Although device configuration is simple, 1) gas pressure introduced to cause plasma must be high due to low plasma efficiency, thus gas incorporation into a thin film is high, 2) deposition rate of the thin film is eventually low due to low plasma efficiency, 3) since high-energy γ electrons (secondary electrons), generated when ion gas impacts the target 1, hit a squarely facing substrate, temperature of the substrate goes up to several hundred degrees (° C.) during deposition, 4) since the target 1 and the substrate are squarely facing, part of ions that impact the target 1 directly hits the substrate (recoil ion), damage to the substrate and compositional shift in multi-component film are disadvantageously caused.
Magnetron sputtering was devised to solve problems of two-pole sputtering. FIG. 21 shows a view showing the principle of a typical planar magnetron sputtering. In FIG. 21, a magnet 4 with north pole 4(N) and south pole 4(S) is shown. Magnetic flux lines 5 are also shown. Direct current (DC) sputtering or alternate current (RF) sputtering is employed depending on type of the applied power supply. As described in two-pole sputtering, high-energy γ electrons generated when ion gas impacts the target 1, hit an facing substrate, thereby principally causing temperature rise in the substrate. However, the γ electrons play an important role in keeping the plasma discharge with ionized gas at high energy. As shown in FIG. 21, the magnet 4 is arranged behind the target 4 to generate a curved magnetic field 5 such that γ electrons discharged from the surface of the target 1 are confined near the target surface to increase the number of collision with atmospheric gas. Therefore, there are the following aspects: 1) improving plasma efficiency by accelerating ionization of atmospheric gas (high-rate sputtering) and 2) restraining temperature rise in the substrate due to impact of high-energy γ electrons to the substrate by a closed migration pathway as shown in the drawing (low temperature sputtering). With arrangement of magnetron, the problem of two-pole sputtering was greatly remedied. However, since the substrate and the target 1 are squarely facing to each other, there is a possibility that part of the curved magnetic field reaches the substrate without closing, and thus 1) injection of γ electrons into the substrate and 2) injection of recoil ions into the substrate cannot be completely restrained. Further, if ferromagnetic material is the target, the magnetic field of magnet goes through part of the ferromagnetic material, thereby preventing a magnetic field from being applied to the surface of the target 1 largely enough to confine γ electrons. Therefore, low-temperature and high-speed sputtering of ferromagnetic material is disadvantageously difficult. However, planer magnetron sputtering, which enables formation of a thin film at a high-speed deposition rate with the comparatively simple structure, is widely used.
Facing target sputtering (see patent literatures 2 and 3) shown in FIG. 22 has been devised to overcome the problems of magnetron sputtering. Two targets 1 are faced to each other such that the magnets 4 with opposite poles are arranged at each back side of the targets. The high-energy γ electrons released from the target surfaces in response to impact of ionized atmospheric gas on the targets generate high-density plasma confined between the facing targets. Since the substrate is placed beside the facing targets outside of the plasma, γ electrons and recoil ions are completely prevented from being injected into the substrate, and thus low temperature sputtering can be achieved. With high-density plasma that confines γ electrons, atmospheric gas pressure can be lowered (down to 10−2 Pa) while performing the discharge and atmospheric gas incorporation into a thin film can be reduced. Therefore, low-temperature sputtering can be performed characteristically when ferromagnetic material is used as a target. Direct current (DC) sputtering or alternate current (RF) sputtering is available depending on the applied power supply.
However, when comparing FIG. 21 and FIG. 22, it can be found that the magnetic field 5 generated by the magnet 4 that is arranged at the rear surface of the target is closed in the planar magnetron sputtering, while magnetic flux lines are closed in the conventional facing target sputtering with opposite pole of magnets arranged between the targets that are facing to each other, as can be seen from the targets, the magnets at the rear surfaces of the targets and the appearance of the magnetic field lines in the conventional sputtering. However, as apparently shown in the drawings, the opposite surface of the magnets cannot form closed magnetic flux lines, and thereby leak of magnetic flux lines occurs. The leak of magnetic field at the rear surface means that magnetic field is reduced by that amount between the facing targets and the magnetic field generated by the magnet cannot be effectively guided to the facing targets, and therefore the magnets are not used efficiently. To reduce the above effect, a thick iron yoke must be placed behind poles opposite to the targets in order to reduce the leaked magnetic field, and thereby the overall structure becomes inevitably large. The magnetic field between the facing targets is required to be approximately 150 to 250 Oe (oersted). A neodymium magnet is used to generate a large magnetic field between the facing targets however, thickness of the magnet must be large enough to prevent the magnetic field from developing effectively in response to occurrence of the leaked magnetic field at the pole opposite to the targets as described above. Further, since saturation magnetization of iron yoke is limited, the iron yoke, if it is made too much thin, is magnetically saturated, and thus the magnetic field is leaked behind the iron yoke. The iron yoke for reducing leaked magnetic field is also required to be designed large in thickness. In the magnetron sputtering shown in FIG. 21, the thickness of magnet and iron yoke can be reduced around 60 mm since the magnetic field is closed both at front and back surfaces of the magnet, while in the conventional composed-mode composed-target sputtering, the thickness of magnet and iron yoke eventually becomes around 80 mm.
Although it is advantageous that damage is small, it is hard to obtain high deposition rate comparing to magnetron sputtering due to positional relation to the substrate, and therefore it is disadvantageous in view of productivity. To increase the deposition rate, if applied power is increased with respect to DC power supply for DC sputtering or AC power supply for RF sputtering, plasma is likely to concentrate between the center portions of targets and thereby causing saturation phenomenon of the applied power and deposition rate cannot be increased due to the saturation.
In contrast, in a facing target sputtering with mixed mode (see patent literature 1) combining facing mode and magnetron mode as shown in FIG. 23, the magnet 4 is arranged at the rear surface of the one target unit 3 at the same position as the magnetron sputtering shown in FIG. 21, while the magnet 4 is arranged at the rear surface of another facing target unit 3 such that the opposite pole is arranged with respect to the one magnet. A back yoke 6 is also shown. A curved magnetic field (magnetron mode) is formed on each of the target surfaces as in the magnetron sputtering, while a vertical magnetic field (facing mode) is formed between the facing targets due to opposite poles. Leak of magnetic flux lines outside of a target holder caused as a problem in the facing mode facing target sputtering in FIG. 22 is resolved by formation of a closed magnetic circuit due to opposite poles as shown in FIG. 23, and thickness of the iron yokes can be sufficiently around the same as in the magnetron sputtering, which is thinner than in the facing target sputtering. The positional relations between the substrate and the facing two targets are the same as the facing target sputtering and the magnetic field is configured hardly to be injected into the substrate, and thus a low-temperature sputtering can be realized.
In the mixed mode, with formation of magnetic field by the magnetron mode and the facing mode, saturation of deposition rate, caused by increasing an applied power to sputtering only based on the facing mode, hardly occurs, and thus a significantly high deposition rate can be advantageously obtained.
However, if the target 1 is ferromagnetic material as shown in FIG. 24, magnetic flux lines 5 of the magnet 4 go through part of the ferromagnetic material and a curved magnetic field on the magnetron mode is hardly applied to the surface of the target, and thereby only the component of vertical magnetic field on the facing mode is applied thereto. A target 1a of ferromagnetic material and weak magnetic flux lines 5a are shown in FIG. 24. If strength of magnets is the same, the magnetic field on the facing mode between ferromagnetic targets is reduced compared to facing target sputtering with facing mode as shown in FIG. 22 in proportion to buried curved magnetic field on the magnetron mode into the ferromagnetic target. A low-temperature high-speed sputtering of ferromagnetic material with high-density plasma confining γ electrons is disadvantageous in performance compared to a pure facing target sputtering with facing mode. It is found from the results of magnetic field simulation that the vertical magnetic field is halved in size compared to that in the case of the single facing mode due to the effect brought by formation of a loop magnetic field. This reduced vertical magnetic field will diminish the effect of confining γ electrons between the targets compared to the effect in the case of the single facing mode. In short, the mixed mode has more effect of low-temperature sputtering than magnetron sputtering, however the effect is slightly less than the single facing mode.
Electronic elements or optical thin films are mostly formed in a multilayer film structure in recent years, and the multilayer film structure is required to be made without breaking vacuum conditions. And, the thin-film includes a variety of materials such as magnetic material, non-magnetic material, metal material, dielectric material, etc. When making the multi-layer film structure by using the facing target sputtering as shown in FIG. 23, facing target cathodes corresponding to each kind of film of the multilayer film are required to be arranged in parallel as shown in FIG. 25. Similarly, magnetron cathodes corresponding to each kind of film of the multilayer film are required to be arranged also in magnetron sputtering shown in FIG. 21. When making by sputtering the multilayer film structure including a mixture of magnetic material, non-magnetic material, metal material, dielectric material, etc. in the same vacuum device, making selective use of sputtering that is effective to each material is required to provide a high-quality multilayer film structure. As described above, the low-temperature sputtering can be realized by the facing target sputtering with facing mode shown in FIG. 22 and the facing target sputtering with mixed mode shown in FIG. 23.
To make a multi-layer film structure with various kinds of materials, respectively different targets and sputtering conditions must be selected. Besides selection of the facing target sputtering shown in FIG. 22 or the magnetron sputtering shown in FIG. 21, a plurality of targets corresponding to the number of film kinds of multilayer film are inevitably arranged in parallel as shown in FIG. 26 or the device with box rotating targets as shown in FIG. 27 (see patent literature 4) must be provided.
Further, in recent years, a flexible device forming a transparent conductor on organic thin-film substrate is desired in a wide range of areas including display elements represented by organic EL elements and solar batteries. For this purpose, sputtering must be applied to an organic thin-film substrate with thermal and physical susceptibilities. Therefore, so-called low-temperature sputtering technology, which causes no damage, is desired. Generally, this means a slow deposition at low deposition rate. On the other hand, however, a high-speed deposition technology is desired from a point of view of productivity.
If applied power is increased to perform a high-speed sputtering, deposition rate is increased. However, atoms and molecules from the target hit the substrate with a large kinetic energy, thereby causing damage to the thin film and the substrate. More particularly, as described above, if magnetron sputtering capable of high-speed sputtering is used, generated γ electrons and recoil gas (generally argon gas) cause damage to the substrate, bring temperature rise in the substrate and cause incorporation of negative ions into the thin-film layer. A solution of these existing contradictory problems has been desired.
In non-patent literature 1 and patent literature 5, when making a transparent conductive film on as-grown film, an initial growth layer is formed by the facing target sputtering and the remaining layers are formed by the magnetron sputtering. In either method of sputtering, a voltage is applied to perform sputtering with the target as a cathode and the vacuum device side as an anode. The initial growth layer of the thin film is made with the facing target sputtering causing little damage, while the remaining layers are made with magnetron sputtering that may cause some damage but can secure high-speed deposition rate depending on the existence of the initial growth layer, which can mitigate damage to as-grown layer. In this manner, a transparent conductive film is formed. A single chamber must accommodate two cathodes and a transfer device including cathodes of facing target sputtering and a substrate transfer mechanism between cathodes of magnetron sputtering, and therefore the increase of chamber capacity causes a problem.